These settlers were followed by people, initially from the Pontic steppe of southern Russia, who knew how to mine for copper and work with gold, and who carried the genetic variant for a blood disorder called haemochromatosis, a hereditary genetic condition so common in Ireland that it is sometimes called Celtic disease.

These people also brought with them the inherited variation that permits the digestion of milk in maturity – much of the world becomes intolerant to the milk sugar lactose after infancy – and they may even have brought the language that became what is now Irish. Some of them, too, had blue eyes.

“There was a great wave of genome change that swept into Europe from above the Black Sea into Bronze Age Europe and we now know it washed all the way to the shores of its most westerly island,” said Dan Bradley, professor of population genetics at Trinity College Dublin.

“And this degree of genetic change invites the possibility of other associated changes, perhaps even the introduction of language ancestral to western Celtic tongues.”

The Dublin team and colleagues from Queens University Belfast report in the Proceedings of the National Academy of Sciences that the two great changes in European prehistory – the emergence of agriculture and the advance of metallurgy – were not just culture shifts: they came with new blood. An earlier population of hunter gatherers was successively overwhelmed by new arrivals. And in Ireland, these new settlers began to define a nation.

But the latest study throws more light on the birth of a nation. All three dead men from Rathlin Island carried what is now the most common type of Irish Y chromosome, inherited only from male forebears. [. . .]

And Lara Cassidy, a researcher in genetics at Trinity College Dublin and another co-author, said “Genetic affinity is strongest between Bronze Age genomes and modern Irish, Scottish and Welsh, suggesting establishment of central attributes of the insular Celtic genome 4,000 years ago.”

The Neolithic and Bronze Age transitions were profound cultural shifts catalyzed in parts of Europe by migrations, first of early farmers from the Near East and then Bronze Age herders from the Pontic Steppe. However, a decades-long, unresolved controversy is whether population change or cultural adoption occurred at the Atlantic edge, within the British Isles. We address this issue by using the first whole genome data from prehistoric Irish individuals. A Neolithic woman (3343–3020 cal BC) from a megalithic burial (10.3× coverage) possessed a genome of predominantly Near Eastern origin. She had some hunter–gatherer ancestry but belonged to a population of large effective size, suggesting a substantial influx of early farmers to the island. Three Bronze Age individuals from Rathlin Island (2026–1534 cal BC), including one high coverage (10.5×) genome, showed substantial Steppe genetic heritage indicating that the European population upheavals of the third millennium manifested all of the way from southern Siberia to the western ocean. This turnover invites the possibility of accompanying introduction of Indo-European, perhaps early Celtic, language. Irish Bronze Age haplotypic similarity is strongest within modern Irish, Scottish, and Welsh populations, and several important genetic variants that today show maximal or very high frequencies in Ireland appear at this horizon. These include those coding for lactase persistence, blue eye color, Y chromosome R1b haplotypes, and the hemochromatosis C282Y allele; to our knowledge, the first detection of a known Mendelian disease variant in prehistory. These findings together suggest the establishment of central attributes of the Irish genome 4,000 y ago.